Combine coverage results, enable nose xunit output.

The coverage results of each python versions are saved in different
coverage files, and combined in the 'coverage' test which is runned last.

Also output the nosetests xml results in different files.
Merge pull request #27 from cdevienne/master

Combine coverage results, enable nose xunit output.
simplify utils.py, add unittests (incomplete)
Merge branch 'master' of https://github.com/Pylons/webtest
Improve readability of webtest.forms.Form.get method (and improve coverage by deleting unneeded files)